As shown in FIG. 1, conventional total knee arthroplasty (TKA) systems typically include a femoral component 500 that is implanted on the distal end of the femur and replaces the bearing surfaces of the femur, a tibial component 502 that is implanted on the proximal end of the tibia and replaces the bearing surfaces of the tibia and meniscus, and a patellar component (not shown) that replaces the articular surface of the patella. In such systems, the femoral component 500 is typically a single solid component. The tibial component 502 may include a tibial base plate (or tray) 502a that is affixed to the bone and a tibial insert 502b that is disposed on the tibial base plate 502a and forms the bearing surfaces of the tibial component 502. Alternatively, the tibial bearing surface may be cemented directly to the bone. In operation, the bearing surfaces of the femoral component 500 articulate against the bearing surfaces of the tibial component 502 as the knee joint moves through a range of motion.
One disadvantage of conventional TKA systems is that the incision must be large enough to accept implantation of the femoral component 500 and the tibial component 502. Another disadvantage is that the femoral component 500 and the tibial component 502 have standard, fixed geometries and are available in a limited range of sizes. As a result, the surgeon may be unable to achieve a fit that addresses each patient's unique anatomy, ligament stability, and kinematics. Additionally, because the conventional implant geometry is fixed, the surgeon may be forced to remove healthy as well as diseased bone to accommodate the implant. Thus, conventional TKA systems lack the flexibility to enable the surgeon to select implant components that are customized to accommodate a patient's unique anatomy and/or disease state.
In an effort to overcome disadvantages of conventional TKA systems, modular TKA knee prostheses comprising multiple components that are inserted separately and assembled within the surgical site have been developed. An example of a modular system is described in U.S. patent application Ser. No. 11/312,741, filed Dec. 30, 2005, published as U.S. Patent Application Publication 2006/0190086, and hereby incorporated by reference herein in its entirety. One disadvantage of such systems is that the modular components, although inserted separately, are connected together inside the patient's body. Thus, the modular components mimic a conventional TKA system, and, as a result, have limitations similar to those of a conventional TKA system. Additionally, because the modular components are fixed together, the components are dependent upon one another in that the selection and placement of one modular component is determined (or constrained) by the selection and placement of another modular component. For example, each modular component may include a connection mechanism (e.g., pins, screws, etc.) designed to mate with a corresponding connection mechanism on another modular component. Because the two components may mate together, the selection and placement of a component is determined and constrained by the selection and placement of the mating component. As a result, the degrees of freedom, interchangeability, and design variability of each modular component are restricted and the final geometry of the assembled component is fixed. Thus, conventional modular implants do not enable the surgeon to vary the placement or geometry of each modular component to best suit each patient's unique anatomy, ligament stability, kinematics, and disease state.
Furthermore, in a number of situations, knee replacement candidates have arthritis in only the medial and patellofemoral compartments, with an intact lateral compartment. If a monolithic component is used to replace only the medial and patellofemoral compartments of the femur and the medial compartment of the tibia (thus, leaving the lateral compartment and cruciate ligaments intact), the surgeon has fewer intra-operative options to best fit the implant to the patient's anatomy.
Efforts have been made to develop multi-compartmental, non-connected systems that do not suffer shortcomings, such as those mentioned above. Such systems can include, for example, non-connected patellofemoral and femoral condyle components. However, with such multi-compartmental, non-connected systems there is a potential that there will not be a smooth transition of the patella or patellar component from the patellofemoral component to the femoral condyle component(s). This can result, for example, in shortened life of the patella or patellar implant or even failure. In addition, it may cause discomfort to the patient.